fix: remove ZWSP sort prefix from Atlas agent name

The \u200B (zero-width space) in AGENT_LIST_SORT_PREFIXES leaked into
the x-opencode-agent-name HTTP header, causing invalid header errors.

Agent ordering is handled by agent-priority-order.ts via numeric order
fields and insertion order — the ZWSP prefix was redundant.

src/shared/agent-display-names.ts

@@ -20,9 +20,7 @@ export const AGENT_DISPLAY_NAMES: Record<string, string> = {
   "council-member": "council-member",
 }
 
-const AGENT_LIST_SORT_PREFIXES: Record<string, string> = {
-  atlas: "\u200B",
-}
+const AGENT_LIST_SORT_PREFIXES: Record<string, string> = {}
 
 function stripAgentListSortPrefix(agentName: string): string {
   return agentName.replace(/^\u200B+/, "")